Hi everyone I'm only a beginner with making cakes I've not done anything for a while now and really wanted to do 2 tier birthday cake .No sure what size cake tins to use can anyone offer advice pls.

What's the best sugarpaste to work with.

What size tine depends on how many people you want to serve.  Under 25 people use an 8" round and a 4 or 5"er.  You will have more than enough cake to serve about 30-35.

Another way to judge is how many servings the recipe you are using serves.  Most cake recipes (scratch or mix) serve about 20. 

A great recipe for you to cover with sugarpaste is my *original* WASC.                                      
http://www.cakecentral.com/recipe/55801/the-original-wasc-cake  recipe  

AS shown in this  ^^  link it is a bit jumbles and hard to read.  I hope you can make it out.     

Found this link that shows the recipe better, along with some of the replies.  Read them also as much more info is there:  http://www.cakecentral.com/recipe/7445/the-original-wasc-cake-recipe      

Thankyou  for  your help.

I cake vouch for the kakeladi recipe. :) awesome. Best part of her recipe is the stuff she wrote about flavoring. I LOVE her vanilla, almond, butter combo----THE BOMB!!!! I find the often recommended recipe has too much almond flavor and it over powers the cake. 

Her cake is a little "denser" than the others, so great for stacking and carving. Kudos to you Kakeladi, and good luck sprinkles!